1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus which is provided with an ultrasonic probe comprising a number of ultrasonic transducers arranged in a specified direction to obtain internal tomographic images of a subject by transmitting and receiving ultrasonic waves from this ultrasonic probe and, more particularly, to an ultrasonic diagnostic apparatus with an improved delay control.
2. Description of the Related Art
An ultrasonic diagnostic apparatus has been used which is capable of facilitating diagnosis of an internal disease of a human body by transmitting ultrasonic waves into a subject, particularly a human body, receiving echo ultrasonic waves reflected from the tissues of the human body by an ultrasonic probe having a number of ultrasonic transducers and displaying an image of an affected part of the human body based on these received ultrasonic waves.
FIG. 16 is a block diagram showing a configuration example of a conventional ultrasonic diagnostic apparatus.
Control means 8 reads out transmission delay control data from transmission delay amount storage means 4 and sets the transmission delay control data in all channels of transmission delay means 3. Then the control means 8 transmits a transmission start signal to the transmission delay means 3. The transmission delay means 3 receives this transmission start signal and generates a transmission pulse at a time based on the preset transmission delay control data. This transmission pulse is converted to a high voltage pulse by a transmission driver 2 and individual transducers 1 are driven with this high voltage pulse.
FIG. 17 is a graphic diagram showing a relationship between arranged ultrasonic transducers and focal positions in a subject.
For example, in case of a linear scan type, a scanning line normal to the direction of the array of transducers is formed as shown in FIG. 17, by ultrasonic waves transmitted from respective transducers 1 shown in FIG. 16. For transmitting ultrasonic waves into a subject so that a focus may be formed at point A shown in FIG. 17, a difference between path OA and path RA (SA) is as shown below, taking into account the timings of transmission pulses corresponding to point R (transducer 1.sub.-- 1) and point S (transducer 1.sub.-- 128). ##EQU1##
Assuming a delay time defined by transmission delay means 3 corresponding to the transducer nearby point 0 as D and the velocity of sound as V, a delay time K defined by the transmission delay means 3 corresponding to points R and S is as given below. ##EQU2##
Ultrasonic waves to be transmitted through each of other transducers can similarly be delayed as long as a time given by the above expression from distance H from point O to the transducer. This is the same with focuses B and C.
FIG. 18 is graphic diagram showing a relationship between transducers 1 and respective delay times.
For transmitting ultrasonic waves so that the focus is positioned respectively at focal points A, B or C shown in FIG. 17, ultrasonic waves are transmitted from each transducer 1 while being delayed for a specified delay time along the curve of focus A, B or C shown in FIG. 18.
Description is continued back to FIG. 16.
Ultrasonic waves reflected from a boundary of tissues in the subject are received again by the ultrasonic transducers 1 and converted to electric signals. The signals received and converted to electric signals are amplified to a specified degree of amplification by a preamplifier 5 and entered into reception delaying/adding means 9. Reception delaying/adding means 9 delays the received signals entered from individual channels as long as specified and adds the received signals from all channels to obtain scanning signals. In the example shown in FIG. 16, reception delay means 10 corresponding to respective channels are provided and control means 8 reads out received delay-controlled data stored in the received delay amount storage means 7 and sets the received delay-controlled data in all reception delay means 10. Reception delay means 10 delays the received signals to be entered as long as a time based on given received delay-controlled data. For example, such representative reception delay means is known as for varying the delay time by selectively connecting input and output taps by a switch through an electromagnetic delay line having a plurality of input and output taps or for converting received signals to digital signals by the A/D converter and delaying the timing by using the shift register or a memory such as an SRAM and a DRAM.
Assuming a distance from the center of scanning to each transducer as H and a focal distance as k, a delay time given to each channel in case of a linear scan type is given as shown below as in case transmission: ##EQU3##
If the focus for reception is set at only the same focus as for transmission, the resolution is satisfactory only in an area near the focus and unsatisfactory in other areas where ultrasonic waves are dispersed. Therefore, a method referred to as "dynamic focus" by which the focal point set for reception is shifted to a farther distant point in sequence in accordance with penetration of ultrasonic waves, that is, the time which has passed after transmission is adapted to obtain a uniform resolution regardless of the penetration of ultrasonic waves. For example, focuses A, B and C shown in FIG. 6 are set with interval d and the focus is shifted by changing the setting to focus B with lapse of time 2d/V after ultrasonic waves reflected at focus A have reached point O and further to focus C with lapse of time 2d/v.
All received signals which are thus time-adjusted in accordance with the time of arrival are added by adding means 11 and a scanning signal thus obtained is sent to the displaying means (not shown) and a tomographic image of an internal part of the subject is displayed.
Along with diffusion of ultrasonic diagnostic apparatuses in recent years, a demand for higher resolution has been intensified and it has been necessary to implement a larger scanning aperture by increasing the number of transducers 1.
The following discusses a consideration for further improvement of resolution by carrying out the dynamic focus setting in the above described conventional example. Generally, improvement of the resolution in the focal area can be achieved by enlarging the scanning aperture or increasing the number of elements. In this case, the difference of path between the ultrasonic waves received at the central point O of scanning (refer to FIG. 17) and those received at end points R and S is large. Accordingly, the delay time D of received signals obtained by the transducers around the center of scanning in the case shown in FIG. 7 should be larger. Though the resolution at the focus is further improved if the the scanning aperture is made large, dispersion of ultrasonic waves at a position away from the focus is larger than that in case of the smaller scanning aperture and, in turn, the resolution deteriorates. Therefore, the distance d between focuses (refer to FIG. 17) need be small and the number of focuses to be set by the dynamic focus setting method need be increased.
As a result of extending the delay time D and reducing the distance d between focuses, in other words, in case of D&lt;2d/V, a problem will occur in the conventional dynamic focus setting method.
The following discusses a case where D&gt;2d/V is satisfied. When, for example, an ultrasonic wave reflected from the focus A shown in FIG. 6 is received at point O, the reflected ultrasonic wave which is then sent toward point R stays at point P. The signal received at point O is delayed only as long as a delay time D (refer to FIG. 18) and must finally be added at the same time as the signals of reflected ultrasonic waves from the focus A which are received by all transducers 1. Because of D&gt;2d/V, however, the delay time of each received signal in reception delay means 10 (refer to FIG. 16) is changed to a delay time (refer to FIG. 18) corresponding to the focus B before the ultrasonic wave reflected from the focus A is received by transducers 1.sub.-- 1 and 1.sub.-- 128 at both ends. The signal of ultrasonic wave reflected from the focus A which is received by transducers 1.sub.-- 1 and 1.sub.-- 128 arranged at, for example, points R and S is delayed as long as a delay time corresponding to the focus B and accordingly a correct focus is not formed to result in a cause of deterioration of the resolution contrary to improvement. The conventional method is therefore restricted by D&lt;2d/V and the improvement of resolution is also limited.
Another problem related to the delay is an unevenness of the velocity of sound in a subject.
For the purpose of comparison, an ultrasonic diagnostic apparatus which is not provided with a feature for compensating wavefront deviation of ultrasonic waves is first described and an ultrasonic diagnostic apparatus provided with the feature for compensating the wavefront deviation is next described.
FIG. 19 is a block diagram of a conventional typical ultrasonic diagnostic apparatus which has been known from the Patent Application Disclosure No. 28989-1978.
Transmission storage means 113 shown in FIG. 19 stores, for example, the transmission delay time data for ultrasonic transducers corresponding to the focal positions shown in FIG. 18. Control means 108 reads out the transmission delay data corresponding to the specified focal positions stored in the transmission storage means 113 and sets them in a group of transmission delay circuits 117. Driving pulses are outputted from the transmission delay circuit group 117 in accordance with the transmission delay data, which have been set at respective timings in response to the differences in the times necessary for the ultrasonic waves transmitted from corresponding transducers 1.sub.-- 1, 1.sub.-- 2, . . . , 1.sub.-- 128 to reach the specified focal positions. These driving pulses are converted to high voltage pulses by a group of transmission circuits 102 and drive corresponding transducers 1.sub.-- 1, 1.sub.-- 2, . . . , 1.sub.-- 128 of a group of transducers 1, thereby ultrasonic waves are generated toward the inside of the subject (not shown). Ultrasonic waves transmitted these transducers 1.sub.-- 1, 1.sub.-- 2, . . . , 1.sub.-- 128 are synthesized to form an ultrasonic beam which is to be focused on the specified focal position in the subject and this ultrasonic beam is transmitted into the subject.
Ultrasonic waves transmitted into the subject are reflected from the boundary of tissues or the like in the subject and received again by transducers 1.sub.-- 1, 1.sub.-- 2, . . . , 1.sub.-- 128 which form the group of transducers 1. These received signals are respectively amplified by preamplifiers which form the group of preamplifiers 103 and entered into the delaying/adding means 7. In this case, the reception storage means 109 stores reception delay data, as shown in FIG. 18 as in case of transmission, corresponding to the focuses shown in FIG. 17 and the reception delay data corresponding to the specified focuses are read out by the control means 108 from the reception storage means 109. Received signals entered into this delaying/adding means 107 are respectively delayed in accordance with reception delay data by the delay line 106 so that the specified focuses are formed in the subject, and added to one another by the adder 120. Only received signals from positions around the focus are emphasized and received signals from other positions are suppressed. Received signals which are outputted from this adder 120 and added to one another are transmitted to the display unit, not shown, and the display unit displays a tomographic image of an inner part of the subject in accordance with these added received signals.
In this case, if the velocity of sound in the subject is uniform, a focus is formed in accordance with a delay time calculated in FIG. 18. However, an actual human body comprises various different systems and substances such as fat, muscles, liver and so forth and it is known that the velocity of sound in fat is 1480 m/sec. substantially smaller than that in other systems and substances such as muscles and a liver as 1570 m/sec.
In other words, there is a problem that, if the delay time is set with the velocity of sound as fixed, the wavefronts of ultrasonic waves transmitted into a human body by the transducers or ultrasonic waves which are reflected to reach the transducers are deviated and are not aligned to result in deterioration of the resolution. In addition, the thickness of the fat layer differs with individual subjects, for example, male and female human bodies and therefore the velocity of sound cannot be involved in advance as a constant factor in calculation.
An idea for forming an ideal focus by detecting and compensating this wavefront deviation has already been proposed in U.S. Pat. No. 4,817,614.
FIG. 20 is a basic configuration of an ultrasonic diagnostic apparatus provided with the feature for compensating the above described wavefront deviation. To avoid an overlapped description, the following sets forth only the points differing from the ultrasonic diagnostic apparatus shown in FIG. 19.
When transducers 1.sub.-- 1, 1.sub.-- 2, . . . , 1.sub.-- 128 are driven by the group of transmission circuits 102, ultrasonic waves are transmitted from these transducers 1.sub.-- 1, 1.sub.-- 2, . . . , 1.sub.-- 128 into a subject. Transmitted ultrasonic waves are reflected from a focal position in the subject while the wavefronts of ultrasonic waves are deviated from one another due to a fat layer near the surface of the subject body. These received signals are respectively amplified by the group of preamplifiers 103, then delayed by the delay line 106 under assumption that, for example, the velocity of sound is fixed, and entered into the time lag detector 121 after sampling with a specified interval of time and A/D conversion. The time lag detector 121 calculates a correlative function of two received signals obtained by adjacent transducers and a time lag, that is, a wavefront deviation of two received signals is obtained from the maximal value of correlative function. An algorithm for detecting this time lag is not the main theme of the present invention and is described in detail in a known example U.S. Pat. No. 4,817,614 and therefore it is omitted from this description.
Though it is described that received signals are entered into the time lag detector 121 after they have been converted to digital signals, a time lag detector capable of carrying out correlative calculation of received signals as are provided. In this case, received signals are respectively sampled by the sample hold circuit, accumulated in the analog memory, and entered into the time lag detector.
When a time lag is thus detected by the time lag detector 121, transmission delay time data and reception delay time data stored respectively in the transmission storing means 113 and the reception storing means 109 are rewritten so that proper focuses are formed by compensating the time lag. Therefore, in transmission and reception of following ultrasonic waves, the delay time in transmission and reception is compensated and a finely adjusted focus is formed despite of unevenness of the velocity of sound.
In a configuration shown in FIG. 20, the memories (transmission storage means 113 and reception storage means 109) for storing delay time data for forming the focus which has been calculated with the uniformity of the velocity of sound are used as the memories for storing delay time data after compensation and therefore it is advantageous in that new memories need not be additionally provided for compensation. In this configuration, however, the contents of the RAM need be rewritten, in turn, for all focuses. Approximately 16 focuses are set for transmission and approximately 64 focuses are set for reception and all data for 128 transducers, that is, a great deal of data need be rewritten for all focuses and therefore electric noise often occurs and it is impossible to rewrite the data during reception of ultrasonic waves. Accordingly, the data need be rewritten by suspending transmission and reception and therefore the frame rate falls down as much as such suspension to result in a problem in use.
Another conventional example and the points to be noted are described below.
FIG. 21 is a basic configuration of another conventional ultrasonic diagnostic apparatus. This conventional example is configured to control the phases of signals in reception. FIG. 22 shows an example of a time waveform of received signal. Differences from the ultrasonic diagnostic apparatus shown in FIGS. 19 and 20 are described below.
A typical time waveform of each received signal is as shown in FIG. 22. This waveform has a convexed envelope with a natural frequency of the transducer as a carrier. For example, assuming that a received signal shown with a solid line and a received signal shown with a broken line which is deviated by approximately 100 n sec. from the former received signal are added to one another, the time lag in this case is approximately 100 n sec. and the distance of forward and backward travels of the sound wave during this time lag is approximately 0.08 mm, substantially smaller than the resolution 1 mm of general ultrasonic diagnostic apparatuses. Generally, it is thought that the time lag as long as approximately .+-.1 cycle of the carrier hardly affects the resolution. However, if there is a phase difference in respective received signals, not only a mere time lag but also such phase difference should be compensated. If the frequency of the carrier is assumed to be 3.5 MHz, the phase of received signal deviates by 3/8 cycles during 100 n sec., and therefore the suppression of these received signals when they are added is large and the signals will have a smaller level after addition. Therefore, to further improve the effect of addition, the phase need be aligned by delaying more finely than approximately .+-.1 cycle. If such phase alignment is performed only by the delay line, the tap pitch on the delay line should be extremely fine and the costs of the delay lines, costs and scale of selector switches and control amount will increase. As seen from the Patent Application Disclosure No. 96286-1979, a group of phase shifters 104 (refer to FIG. 21) are connected to the signal lines of transducers 1.sub.-- 1, 1.sub.-- 2, . . . , 1-128 and the outputs of these phase shifters are selectively connected to the taps of the delay line through selector switches 105. Based on this configuration, the above described fine phase alignment can be carried out by the group of phase shifters 104 and the time lag can be roughly adjusted by the delay line 106 which can be formed in an appropriate scale.
In case of reception for which, for example, the focuses are respectively set at focal points a1, a2, . . . , a5, b0, b1, . . . , c5 as shown in FIG. 23, the input taps of the delay line 106 are selected for focuses a1.about.a5 located in the area of zone A by controlling the selector switches 105 to match the difference of delay time of point a3 and the positions of these taps are finely adjusted by phase shifters 104 to meet the focuses, respectively. Similarly, the input taps of the delay line 106 are selected by controlling the selector switches 105 to meet the difference of delay time at point b3 for setting the focuses to points b0.about.b5 in the area of zone B and that at point c3 for setting the focuses to points c0.about.c5 in the area of zone C, and the tap of these positions are finely adjusted by phase shifters 104 to meet the focuses, respectively.
An electromagnetic delay line is widely used as the delay line. Such delay line does not always ensure an ideal delay line and may include an error. When an ultrasonic beam is to be deflected as in sector scanning or increasing the scanning aperture to obtain a high resolution, a delay time through the delay line often increases to reach a duration of more than 10 .mu.sec., maximum. In this case, even though a difference of delay time from each input tap to an output is 1%, the time lag of received signals will be finally more than 100 .mu.sec. and the phase will be largely deviated. In the conventional example, therefore, the delay time obtained from each input tap to an output of the delay line mounted on the apparatus has been measured, phase control data has been prepared based on the measured data and the data obtained has been stored in the reception storage means 109. The actual apparatus usually has approximately eight zones and accordingly the taps of the delay line are selected in eight different ways. However, there are 64 or more focal points which can be set and therefore the phase control data of 64 focal points need be corrected for each of eight zones. These data are not hardware-compatible because the delay line differ with the type of apparatus. In other words, it is necessary for each apparatus to correct all phase control data and store it in storage means, thus imposing a substantial load on manufacturers.
As described above, there have been many problems with respect to storage and reentry of transmission delay time data and reception delay time data.